CN101493570B

CN101493570B - Objective lens and optical pickup device

Info

Publication number: CN101493570B
Application number: CN2008101376401A
Authority: CN
Inventors: 富田高弘; 山形道弘; 金马庆明; 林克彦
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 2008-01-23
Filing date: 2008-07-01
Publication date: 2012-01-04
Anticipated expiration: 2028-07-01
Also published as: JP2009199708A; CN101493570A

Abstract

In order that thickness reduction, weight reduction, and mass productivity improvement should be achieved in an objective lens even in the case where NA is high, an objective lens according to the present invention is a bi-convex single lens having at least one aspheric surface, and satisfies conditions: (1) 3.5<DH-S/DH-H'<4.2; (2) 3.5<DH'-T2/DT1-H<50; (3) 0.9<d/f<1.1 (NA>=0.85). Here, DH-S is a distance on the optical axis from a front principal point H to a focal point S on an optical information recording medium side; DH-H'is a distance on the optical axis from the front principal point H to a rear principal point H'; DH'-T2 is a distance on the optical axis from the rear principal point H' to an intersecting point T2 of the optical axis and an optical information recording medium side surface of the objective lens; DT1-H is a distance on the optical axis from an intersecting point T1 of the optical axis and a light source side surface of the objective lens to the front principal point H; d is a thickness on the optical axis; and f is a focal length.

Description

Objective lens and optical pickup device

Technical Field

The present invention relates to an objective lens for performing recording, reproduction, and deletion of information on an optical information recording medium, and an optical pickup apparatus employing the objective lens.

Background

In the conventional art, an optical disc unit capable of writing information onto an optical disc such as a CD medium and a DVD medium, reading information from the optical disc, and deleting information recorded on the optical disc is widely used. In such an optical disc unit, an optical pickup device including an objective lens is employed (see, for example, japanese laid-open patent publication No. 2003-91854).

In recent years, a Blu-ray Disc (registered trademark) using blue laser light has been developed, and has made it necessary to have an objective lens with a very high numerical aperture (NA ═ 0.85). When such an objective lens is to be realized, the thickness of the objective lens generally needs to be significantly increased. This causes an increase in the load of the actuator for driving the objective lens. Further, a problem also arises in that the high NA causes significant off-axis aberration and coma generated by decentering. Further, in such an objective lens having a very high NA, the tilt angle of the lens surface is increased. This causes difficulties in the manufacture of a molding die for objective lens manufacture and in molding of the objective lens.

Thus, an object of the present invention is to provide an objective lens in which thickness reduction and weight reduction can be achieved even in the case of a very high NA while obtaining satisfactory mass productivity.

Disclosure of Invention

In order to perform at least one of recording, reproducing, and deleting of information on the optical information recording medium, the objective lens according to the present invention is for condensing light onto a recording surface of the optical information recording medium. The objective lens is a biconvex singlet lens having at least one aspherical surface, and satisfies the following conditions: 3.5 < D_H-S/D_H-H’＜4.3 (1)3.5＜D_H’-T2/D_T1-H< 50 (2)0.9 < D/f < 1.1 (3) (here, numerical aperture NA on the optical information recording medium side of the objective lens satisfies NA ≧ 0.85) where D_H-SIs a distance [ mm ] from a front principal point H of the objective lens to a focal point S of the objective lens on the optical information recording medium side on an optical axis]，D_H-H’Is the distance [ mm ] from the front principal point H of the objective lens to the rear principal point H' of the objective lens on the optical axis]，D_H’-T2Is a distance [ mm ] from the rear principal point H' of the objective lens on the optical axis to an intersection T2 of the optical axis of the objective lens and the surface on the optical information recording medium side]，D_T1-HIs a distance [ mm ] from an intersection point T1 of the optical axis of the objective lens and a light source side surface to the front principal point H of the objective lens on the optical axis]And d is the thickness of the objective lens on the optical axis [ mm ]]And f is the focal length of the objective lens [ mm ]]。

Alternatively, the objective lens according to the present invention is a biconvex singlet lens having at least one aspherical surface, and satisfies the following condition: 0.8 < R1/R1 < 0.85 (4)0.9 < d/f < 1.1 (3) (here, the numerical aperture NA of the optical information recording medium side of the objective lens satisfies NA ≧ 0.85), wherein R1 is the paraxial radius of curvature [ mm ] of the objective lens on the light source side, R1 is the effective radius [ mm ] of the objective lens on the light source side, d is the thickness [ mm ] of the objective lens on the optical axis, and f is the focal length [ mm ] of the objective lens.

Further, an optical pickup apparatus according to the present invention is used for at least one of recording, reproducing, and deleting of information on an optical information recording medium, and includes a light source and a condensing optical system including any one of the above-described objective lenses for condensing a light beam emitted from the light source onto a recording surface of the optical information recording medium.

According to the present invention, even when NA is equal to or higher than 0.85, thickness reduction and weight reduction can be achieved in the objective lens, and also mass productivity thereof is improved.

These and other objects, features, aspects and effects of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings.

Drawings

Fig. 1 is a schematic configuration diagram showing an optical pickup apparatus according to embodiment 1 of the present invention; FIG. 2 is an optical path diagram of the objective lens shown in FIG. 1; FIG. 3 is a schematic structural diagram showing a computer system according to embodiment 2 of the present invention; fig. 4 is a block diagram showing a schematic structure of the optical disc drive shown in fig. 3; fig. 5 is a longitudinal aberration diagram of an objective lens according to numerical example 1; fig. 6 is a lateral aberration diagram of an objective lens according to numerical example 1; fig. 7 a longitudinal aberration diagram of an objective lens according to numerical example 2; fig. 8 is a lateral aberration diagram of an objective lens according to numerical example 2; fig. 9 is a longitudinal aberration diagram of an objective lens according to numerical example 3; fig. 10 is a lateral aberration diagram of an objective lens according to numerical example 3; fig. 11 is a longitudinal aberration diagram of an objective lens according to numerical example 4; and fig. 12 is a lateral aberration diagram of the objective lens according to numerical example 4.

Detailed Description

(embodiment 1) fig. 1 is a schematic configuration diagram showing an optical pickup device according to embodiment 1 of the present invention.

The optical pickup device according to embodiment 1 includes a light source 1, a collimator lens 3, a prism 4, an objective lens 5, and an actuator 7.

The light source 1 is composed of, for example, a semiconductor laser, and emits a light beam 2 having a wavelength in the range of 390nm to 420 nm. The light beam 2 emitted by the light source 1 is converted into an approximately parallel light beam by the collimator lens 3. The light emitted from the collimator lens 3 is refracted by a prism 4 in a direction perpendicular to the optical axis of the light source 1, and then is condensed onto an optical information recording medium 6 through an objective lens 5.

The objective lens 5 is connected to an actuator 7 so that its central axis approximately coincides with the optical axis of the light refracted by the prism 4. Further, the objective lens 5 is movable in a direction perpendicular to the optical axis of the incident light by an actuator 7. Thus, even when the wavelength of the laser light changes and thus the light beam becomes divergent or convergent, the positional deviation of the spot in the tracking direction on the optical information recording medium 6 can be corrected, that is, tracking control can be performed.

Fig. 2 is an optical path diagram of the objective lens 5 shown in fig. 1. The symbols in fig. 2 are defined as follows. S is a focal position on the optical information recording medium side in the case where a parallel light beam enters from the light source side, T1 is an intersection point of a surface on the light source side of the objective lens and the optical axis, T2 is an intersection point of a surface on the optical information recording medium side of the objective lens and the optical axis, H is a front principal point of the objective lens, H' is a rear principal point of the objective lens, d is a thickness of the objective lens on the optical axis, f is a focal length of the objective lens, R1 is a paraxial curvature radius of the objective lens on the light source side, R1 is an effective radius of the objective lens on the light source side, R2 is an effective radius of the objective lens on the optical information recording medium side, and α is a maximum angle formed between incident light and a normal line passing through a point on the light source side surface of the objective lens, a height from the optical axis to a point on the light source surface. The effective radius refers to a radius of a cross section of a light beam of NA (0.85 in the case of Blu-ray Disc (registered trademark)) that satisfies requirements of an optical pickup apparatus.

Hereinafter, a condition for satisfying the objective lens according to the present embodiment is described. In the following description, a number of conditions are set forth. It is preferable to form the objective lens so as to satisfy the conditions as much as possible. However, an objective lens that satisfies any one of the following conditions and achieves an effect corresponding to the condition can be obtained.

More preferably, the objective lens according to the present embodiment simultaneously satisfies the following conditions. 3.5 < D_H-S/D_H-H’＜4.3 (1)3.5＜D_H’T2/D_T1-H< 50 (2)0.9 < D/f < 1.1 (3) (here, numerical aperture NA on the optical information recording medium side of the objective lens satisfies NA ≧ 0.85) where D_H-SIs the distance [ mm ] from the front principal point H of the objective lens on the optical axis to the focal point S on the optical information recording medium side of the objective lens]，D_H-H’Is the distance [ mm ] from the front principal point H of the objective lens to the rear principal point H' of the objective lens on the optical axis]，D_H’-T2Is a distance [ mm ] from a rear principal point H' of the objective lens on the optical axis to an intersection T2 of the optical axis of the objective lens and the surface on the optical information recording medium side]And D_T1-HIs the distance [ mm ] from the intersection point T1 of the optical axis of the objective lens and the surface on the light source side to the front principal point H of the objective lens on the optical axis]。

When the conditions (1) to (3) are satisfied, even in the case where the objective lens has a high NA, it is possible to make the objective lens thin and secure a required working distance. Further, it is possible to suppress occurrence of off-axis aberration and coma generated by decentering or the like. In contrast, when each value falls outside the range of the conditions (1) to (3), it is difficult to simultaneously achieve reduction in thickness and weight of the objective lens and suppression of aberration.

Further, it is more preferable that the objective lens according to the present embodiment simultaneously satisfies the following conditions, instead of or in addition to the above-described conditions (1) to (3). 0.8 < R1/R1 < 0.85 (4)0.9 < d/f < 1.1 (3) (here, the numerical aperture NA on the optical information recording medium side of the objective lens satisfies NA ≧ 0.85)

When the conditions (3) and (4) are satisfied, even in the case where the objective lens has a high NA, the thickness and weight of the objective lens can be reduced, and the degradation of the convergence characteristics caused by the occurrence of off-axis aberration and coma aberration due to decentering can be suppressed. When R1/R1 is less than 0.8, the maximum inclination angle α becomes very large, thereby causing significant difficulties in the fabrication of a press mold and in the molding of a lens. Further, when R1/R1 is larger than 0.85, occurrence of coma generated by decentering increases.

Further, it is preferable that the objective lens according to the present embodiment satisfies the following conditions. 0.75 < r2/r1 < 0.8 (5)

When r2/r1 is set to be larger than 0.75, light energy concentration on the medium-side surface of the objective lens can be reduced. Therefore, even when light having high energy enters the objective lens, temperature rise can be minimized, thereby reducing degradation of optical characteristics caused by the temperature rise.

As a material for forming the objective lens, plastic resin or glass is used. The above-mentioned advantage of reducing the concentration of light energy is particularly advantageous in the case of plastics. Specifically, when light having a wavelength of about 400nm is used, for example, in the case of a Blu-ray Disc (registered trademark), since light energy is concentrated on the objective lens, a plastic material can be decomposed. However, when r2/r1 is set to be larger than 0.75 according to condition (5), concentration of optical energy can be reduced so that reliability and durability of the objective lens can be improved.

When the optical energy is highly concentrated, the temperature rise of the objective lens becomes larger irrespective of the material of the objective lens, resulting in a problem of focus position deviation. However, according to the condition (5), the concentration of light energy can be reduced, whereby the stability of light condensing performance can be ensured.

Here, when r2/r1 is equal to or less than 0.75, the effect of reducing light energy concentration cannot be satisfactorily obtained. In contrast, when r2/r1 is equal to or greater than 0.8, the occurrence of coma aberration with respect to decentering at the time of molding increases.

Further, it is preferable that the objective lens according to the present embodiment satisfies the following conditions. Alpha < 60 DEG < 65 DEG (6)

When the condition (6) is satisfied, practical design of the objective lens, fabrication of the molding die, and molding using the molding die can be practically performed, thereby improving mass productivity. In contrast, when the maximum inclination angle α falls outside the range of the condition (6), difficulty occurs in the fabrication and molding of the molding die.

Further, it is preferable that the objective lens according to the present embodiment satisfies the following conditions. WD/(d +2 × r1) ≧ 0.12 (7) where WD is the working distance of the objective lens (that is, the distance from the optical information recording medium-side surface of the objective lens to the surface of the optical information recording medium).

As shown in fig. 1, in order that the height (horizontal dimension in fig. 1) of the optical pickup device should be reduced, the optical path is bent at a right angle by using a prism 4. In this case, the height of the optical pickup device is limited by the sum of the thickness of the objective lens 5 and the height of the prism surface necessary for refracting the light beam incident on the objective lens 5, the diameter of which is equal to the effective diameter on the light source side of the objective lens, that is, by the sum (d +2 × r1) of the thickness d and the effective diameter on the light source side (2 × r 1). When compatibility (e.g., compatibility between CDs/DVDs) is required for optical discs of different standards, the working distance WD, i.e., the distance between the objective lens 5 and the surface of the optical disc, needs to be increased. However, as WD increases, the effective radius r2 on the optical information recording medium side increases. As r2 increases, r1 also increases, causing the height of the optical pickup device to increase.

Therefore, with the use of WD/(d +2 × r1) as an index, it is possible to contribute to a reduction in height of the optical pickup apparatus when the objective lens is formed in such a manner that the numerical value of the index should become larger. Specifically, as shown in condition (7), it is more preferable that the value WD/(d +2 × r1) be equal to or greater than 0.12. Further, in order to further reduce the height of the optical pickup device, it is preferable that WD/(d +2 × r1) be equal to or greater than 0.14. In contrast, when the value WD/(d +2 × r1) is less than 0.12, only a small contribution can be made to the reduction in height of the optical pickup.

(embodiment 2) fig. 3 is a schematic configuration diagram showing a computer system according to embodiment 2 of the present invention.

The computer system 10 includes a main body 11, a liquid crystal display 12 as an output device, and a keyboard 13 as an input device. Further, the main body 11 includes a CPU 11a and an optical disk drive 11 b.

Fig. 4 is a block diagram showing a schematic structure of the optical disk drive 11b shown in fig. 3.

The optical disc drive 11b includes the optical pickup 111, the interface 112, the motor 114, the turntable 116, and the clamper 115 according to embodiment 1. In fig. 4, the optical disc 113 is placed on the turntable 116. The interface 112 of the optical disk drive 11b is connected to the CPU 11a through a signal line 11 c.

The CPU 11a transmits various control signals to the optical pickup 111 and the motor 114 through the interface 112. In accordance with the control signal, the motor 114 drives and rotates the optical disk 113 which raises the clamper 115 fixed on the turntable 116. On the other hand, the optical pickup 111 performs reading, writing, and deletion of data on the recording layer of the optical disc 113 in accordance with various control signals from the CPU 11 a.

The optical disc drive 11b constituting the computer system 10 according to embodiment 2 employs the optical pickup device 111 described in embodiment 1. Therefore, the size in the optical disc drive 11b in the optical axis direction of the objective lens is reduced as compared with the conventional optical disc drive. Accordingly, the computer system 10 can be constructed compactly.

Here, embodiment 2 has described an exemplary case of an optical disk drive included in a computer system. However, the objective lens and the optical pickup device according to embodiment 1 can be applied to any information system that stores information, such as an optical disc player, an optical disc recorder, a car navigation system, a writing system, a data server, an AV component, and a vehicle.

(numerical example) in which the objective lens according to embodiment 1 isExamples of values actually implemented are described below. In the numerical example, the length unit related to the lens size in each table is "mm". Further, in those numerical examples, the aspherical shape is defined by the following formula.

Here, the meaning of each symbol is as follows. h is a height from the optical axis, X is a distance from a point on the aspherical surface to a tangent plane at an aspherical vertex, the height of the aspherical surface from the optical axis is h, C is a curvature of the aspherical vertex at the jth surface of the objective lens (C ═ 1/R when a curvature radius of the aspherical vertex at the jth surface of the objective lens is represented by R), K is a conic constant of the jth surface of the objective lens, and a is an nth aspherical coefficient of the jth surface of the objective lens.

Table 1 shows lens data of the objective lenses according to numerical examples 1 to 4 and values corresponding to individual conditions. In table 1, λ denotes a design wavelength, and n denotes a refractive index of a lens material with respect to light having the design wavelength λ.

TABLE 1 (lens data andvalues corresponding to individual conditions)

Tables 2 to 5 show aspherical data of the objective lenses according to numerical examples 1 to 4.

TABLE 2 (numerical example 1)

	Light source side surface (first surface)	Optical information recording medium side surface (second surface)
			R	0.976456	-3.453415
K	-0.9759662	-142.4346
			A4	0072677311	0.23396804
A6	0.019913925	-0.60371079
			A8	-0.003280565	1.3104318
A10	0.037635639	-2.3369295
			A12	-0.07947901	2.6070762
A14	0.11397914	-1.5876507
			A16	-0.10277546	0.40283933
A18	0.05421167	-
			A20	-0.013127114	-

TABLE 3 (numerical example 2)

	Light source side surface (first surface)	Optical information recording medium side surface (second surface)
			R	1.267711	-44.8376
K	-0.9781575	-26795.78
			A4	0.035822796	0.1082869
A6	0.0055122	-0.18078275
			A8	-8.89971E-05	0.27333919
A10	0.004397701	-0.3028286
			A12	-0.006375722	0.19845835
A14	0.005824615	-0.069928387
			A16	-0.003192219	0.010235832
A18	0.001020656	-
			A20	-0.00015437	-

TABLE 4 (numerical example 3)

	Light source side surface (first surface)	Optical information recording medium side surface (second surface)
			R	1.802685	-47.52278
K	-0.9563194	-14670.65
			A4	0.011955319	0.04105487
A6	0.00084973	-0035017292
			A8	0.00013211	0.02486749
A10	6.95402E-05	-0.013311584
			A12	-8.33011E-05	0.004353366
A14	4.90683E-05	-0.000781434
			A16	-1.57705E-05	5.90785E-05
A18	2.75935E-06	-
			A20	-2.13751E-07	-

TABLE 5 (numerical example 4)

	Light source side surface (first surface)	Optical information recording medium side surface (second surface)
			R	0.966362	-4.095634
K	-0.97261	-155.7852
			A4	0.074242747	0.23736105
A6	0.023283506	-0.63818663
			A8	-0.009256113	1.4606853
A10	0.051302282	-2.4381089
			A12	-0.09490531	2.4382605
A14	0.12268312	-1.1315584
			A16	-0.10387775	0.29465413
A18	0.05455745	-
			A20	-0.013600226	-

Fig. 5, 7, 9 and 11 are longitudinal aberration diagrams of objective lenses according to numerical examples 1, 2, 3 and 4, respectively. Further, fig. 6, 8, 10 and 12 are lateral aberration diagrams of the objective lenses according to numerical examples 1, 2, 3 and 4, respectively.

As described above, each of the objective lenses according to numerical examples 1 to 4 is designed to be able to satisfy the above conditional expressions. Therefore, according to the present invention, even in the case where the NA is very high, the thickness and weight of the objective lens can be reduced and mass productivity can be improved. Further, according to the present invention, it is possible to suppress occurrence of off-axis aberration and coma aberration generated by decentering.

The present invention has been described above in detail. However, the description given above is merely an illustrative example of the present invention from all viewpoints, and does not limit the scope of the present invention. It should not be emphasized too strongly that various modifications and changes may be made without departing from the scope of the invention.

Claims

1. An objective lens for converging light onto a recording surface of an optical information recording medium to perform at least one of recording, reproducing, and erasing of information on the optical information recording medium,

the objective lens is a biconvex singlet lens having at least one aspherical surface, and satisfies the following conditions:

3.5＜D_H-S/D_H-H’＜4.3 (1)

3.5＜D_H’-T2/D_T1-H＜50 (2)

0.9＜d/f＜1.1 (3)

here, the numerical aperture NA on the optical information recording medium side of the objective lens satisfies NA ≧ 0.85,

wherein,

D_H-Sis a distance from a front principal point H of the objective lens to a focal point S of the objective lens on the optical information recording medium side on an optical axis, and has a unit of mm,

D_H-H’is the distance on the optical axis from the front principal point H of the objective lens to the rear principal point H' of the objective lens, in mm,

D_H’-T2is a distance in mm from the rear principal point H' of the objective lens on the optical axis to an intersection T2 of the optical axis of the objective lens and the surface on the optical information recording medium side,

D_T1-His a distance in mm from an intersection point T1 of the optical axis of the objective lens and a light source side surface to the front principal point H of the objective lens on the optical axis,

d is the thickness of the objective lens on the optical axis, in mm, and

f is the focal length of the objective lens in mm.

2. The objective of claim 1, wherein the following condition is satisfied:

0.75＜r2/r1＜0.8 (5)

wherein,

r1 is the effective radius of the objective lens on the light source side, in mm, and

r2 is the effective radius of the objective lens on the optical information recording medium side, and has a unit of mm.

3. The objective of claim 2, wherein the objective consists of a plastic material.

4. The objective of claim 1, wherein the objective consists of a glass material.

5. The objective of claim 1, wherein the following condition is satisfied:

60°＜α＜65° (6)

wherein,

α is a maximum angle formed between incident light and a normal line passing through a point on the light source side surface of the objective lens, a height from the optical axis to the point on the light source side surface being equal to or smaller than an effective radius of the light source side.

6. An optical pickup apparatus for performing at least one of recording, reproducing, and deleting of information on an optical information recording medium, comprising:

a light source; and

a condensing optical system including an objective lens for condensing a light beam emitted from the light source onto a recording surface of the optical information recording medium, wherein

3.5＜D_H-S/D_H-H’＜4.3 (1)

3.5＜D_H’-T2/D_T1-H＜50 (2)

0.9＜d/f＜1.1 (3)

here, the numerical aperture NA of the objective lens on the optical information recording medium side satisfies NA ≧ 0.85,

here, ,

D_H’-T2is a medium from the rear principal point H' of the objective lens to the optical axis of the objective lens and the optical information recording medium on the optical axisThe distance of intersection T2 of the mass-side surface, in mm,

d is the thickness of the objective lens on the optical axis, in mm, and

f is the focal length of the objective lens in mm.

7. An optical pickup device according to claim 6, wherein the objective lens satisfies the following condition:

0.75＜r2/r1＜0.8 (5)

wherein,

8. An optical pickup apparatus according to claim 7, wherein said objective lens is composed of a plastic material.

9. An optical pickup apparatus according to claim 6, wherein said objective lens is composed of a glass material.

10. An optical pickup device according to claim 6, wherein the objective lens satisfies the following condition:

60°＜α＜65° (6)

wherein,

α is a maximum angle formed between incident light and a normal line passing through a point on the light source side surface of the objective lens, a height from the optical axis to the point on the light source side surface being equal to or smaller than the effective radius of the light source side.

11. An optical pickup device according to claim 6, wherein the following condition is satisfied:

WD/(d+2×r1)≥0.12 (7)

wherein,

WD is a distance from the surface of the objective lens on the optical information recording medium side to the surface of the optical information recording medium, and is in mm,

d is the thickness of the objective lens on the optical axis, in mm, and

r1 is the effective radius of the objective lens on the light source side, in mm.